1. Field of the Invention
The present invention relates to a method for controlling the pressure prevailing in a simulated moving bed separation system through global control of the inlet/outlet flow rates.
2. Description of the Prior Art
Separation or fractionation processes based on (simulated moving bed) continuous chromatography are most often implemented in a device comprising a set of n chromatographic columns or column sections mounted in series (generally forming an open or a closed loop). A porous solid of well-determined grain size, distributed in different beds, constitutes the stationary phase.
Injection points intended for continuous injection of a solution referred to as feedstock, consisting of a mixture of compounds to be separated dissolved in a suitable fluid, and of an eluent, and fluid draw-off points are distributed along this loop. A flow referred to as raffinate (R), which contains the component that is the least readily retained by the stationary phase, is recovered downstream from a feedstock injection point. Another flow referred to as extract Ex, which contains the component that is the most readily retained by the stationary phase is recovered upstream from this feedstock injection point. An identical liquid flow passes through all the columns or column sections of the same zone. The raffinate flow rate is equal to the sum of the inlet flow rates minus the extract flow rate. In addition to these controlled flow rates, there is a recycle flow rate Q.sub.Re. Several working zones are thus defined, each one defined by an injection and a draw-off point.
The injection and draw-off points are shifted at regular or non-regular time intervals in the direction of flow. The time interval between two injection/draw-off point shifts is referred to as at period.
Separation systems of this type are described for example in U.S. Pat. No. 2,985,589 and 4,402,832 and in pending U.S. Pat. application Ser. No. 09/097,590, filed by the assignee.
The inlet/outlet flow rates can take very different values according to the separations performed. It is well-known that the value of these flow rates (feedstock/extract for example) plays a significant part in the operation of the separation system.
The feedstock, eluent, extract and raffinate flow rates are denoted by Q.sub.Feedstock, Q.sub.El, Q.sub.Ext, Q.sub.Raf.
On an average in time, the sum (E) of the two inlet flow rates (feedstock and eluent) and the sum (S) of the two outlet flow rates (extract and raffinate) must be strictly identical, i.e. EQU E=S,i.e. Q.sub.Feedstock +Q.sub.El =Q.sub.Ext +Q.sub.Raf.
To that effect, it is customary to control the flow rate of three of the four inlet/outlet flows of the system, the fourth one being controlled so as to maintain the pressure constant at a given point of the system. Such a technique is used and described for example in U.S. Pat. No. 3,291,726 and EP Pat. No. 0,586,385.
Selection of the pressure-controlled flow rate can be critical. In particular, if the flow rate selected is low, the relative variations of this flow rate, imposed by control, can be high in relation to the average value thereof, which may generate an unsatisfactory behavior likely to modify the inlet/outlet flow rate equilibrium. The pressure-controlled flow rate may therefore have to be changed according to the separations performed.